1. Field of the Invention
The present invention relates to a linear electromagnetic actuator comprising a permanent magnet and a coil which face each other to displace the permanent magnet by the thrust force generated by the current allowed to flow through the coil.
2. Description of the Related Art
In a linear electromagnetic actuator of the movable magnet type, a permanent magnet and a coil which face each other displace the permanent magnet by the thrust force which is generated based on the Fleming's left hand rule when the current is allowed to flow through the coil (see Japanese Laid-Open Patent Publication Nos. 5-227729 and 10-290560).
In the linear electromagnetic actuator disclosed in Japanese Laid-Open Patent Publication Nos. 5-227729 and 10-290560, the permanent magnet is fixed to a first yoke, and the first yoke is supported by a slider. Meanwhile, the coil is fixed to a second yoke, and the second yoke is supported by a guide rail. Further, a guide member is interposed between the slider and the guide rail.
A magnetic path passing through the permanent magnet, the first yoke, the clearance between the first yoke and the second yoke, the second yoke, the coil, the gap between the coil and the permanent magnet, and the permanent magnet, is formed by the permanent magnet in the linear electromagnetic actuator. When the current is allowed to flow through the coil, a magnetic path of the coil, the gap between the coil and the permanent magnet, the permanent magnet, the first yoke, the clearance between the first yoke and the second yoke, the second yoke, and the coil, is also formed. Accordingly, the thrust force, which acts on the permanent magnet, is generated depending on the direction of the current and the directions of the respective magnetic fluxes passing through the respective magnetic paths. The permanent magnet, the first yoke, and the slider are displaced and guided by the guide member.
When attempting to reduce the thickness of the linear electromagnetic actuator, which is disclosed in Japanese Laid-Open Patent Publication Nos. 5-227729 and 10-290560, the cross-sectional area of the magnetic path is decreased at the first yoke and the second yoke, and local magnetic saturation occurs in the first yoke and the second yoke. As a result, leakage magnetic flux is generated from the first yoke and the second yoke, and the thrust force, which acts on the permanent magnet, is lowered. Further, due to the leakage magnetic flux, some kind of malfunction may occur in the electronic apparatus and the electronic circuit disposed around the linear electromagnetic actuator. Also, attraction of a magnetic member disposed around the linear electromagnetic actuator to the linear electromagnetic actuator may occur as well as malfunction of the electronic circuit and the encoder arranged in the linear electromagnetic actuator.
Further, dust, which is composed of the magnetic member, may be adhered to the guide member by the magnetic flux passing through the magnetic path, thereby increasing the sliding resistance of the guide member with respect to the slider. As a result, it is difficult to smoothly displace the slider, and dust escaping out from the linear electromagnetic actuator may pollute the surrounding environment.